1. Field of the Invention
The present invention relates to a direct-current (DC) power conversion circuit, and more particularly, to a DC-to-DC power conversion circuit with constant current output.
2. Description of the Prior Art
Light emitting diodes (LEDs) have been widely used in indicating lamps and display devices of information, communications and consumer electronic products, such as traffic light indicators. Unlike incandescent lamps, the LEDs are luminescent devices and have advantages of low power dissipation, long life, extremely short warm-up time and fast reaction. Besides, the LEDs are light, resistant to vibration and easy to mass-produce. Regarding physical characteristics of the LED, luminosity is proportional to LED current, and the LED current increases exponentially with a forward bias. That is, the larger the current that flows through the LED, the higher the luminosity the LED emits. Furthermore, the LED current varies exponentially with ambient temperature.
Specific applications, such as backlights of LCDs and traffic light indicators, require the LED to luminesce stably. Therefore, these applications generally require a voltage conversion circuit to control the LED current, so as to maintain the LED luminosity.
Please refer FIG. 1, which is a schematic diagram of a DC/DC voltage converter 10 according to the prior art. The DC/DC voltage converter 10 includes a direct-current (DC) voltage source 100, a switch transistor 102, a resistor 104, a diode 106, an inductor 108, a capacitor 110, a load circuit 112, a control signal generator 114, and a voltage regulator 116. The control signal generator 114 generates a control signal Sc according to a current of the resistor 104, and thereby the voltage regulator 116 regulates a voltage level of the control signal Sc, so as to control the switch transistor 102. The working principle of the DC/DC voltage converter 10 is explained as follows. As the switch transistor 102 is turned on, the diode 106 bears a reverse bias, and thereby a loop L1 becomes open. As a result, the DC voltage source 100 charges the inductor 108, and meanwhile, an increasing current flows into the resistor 104. As the current of the resistor 104 increases to a specific level, the control signal generator 114 turns off the switch transistor 102 with the control signal Sc. Once the switch transistor 102 is turned off, the inductor 108 accordingly generates an inverse-polarity voltage, so that the diode 106 bears a forward bias and loop L1 begins conducting current. Meanwhile, the current of the resistor 104 starts to decrease gradually. To sum up, the DC/DC voltage converter 10 maintains current output to the load circuit 112 in a tight range by alternatively turning the switch transistor 102 on and off.
In the DC/DC voltage converter 10, the control signal generator 114 and the other devices use a same grounding node, GND, such that the switch transistor 102 and the control signal generator 114 are situated at different voltage levels. The switch transistor 102 operates at a voltage high side, whereas the control signal generator 114 operates at a low side, and the control signal Sc generated is at the voltage low side as well. As a result, the DC/DC voltage converter 10 depends on the voltage regulator 116 to pre-regulate the control signal Sc to an appropriate voltage level for the switch transistor 102. That is, the voltage regulator 116 raises the voltage level of the control signal Sc to the voltage high side. However, the voltage regulator 116 is typically a transformer device, resulting in high circuit complexity and area in the DC/DC voltage converter 10.